The present invention generally relates to a semiconductor device including a variable capacitor using a PN junction. More particularly, the present invention relates to a method for increasing a variation range of a capacitance.
Conventionally, a variable capacitor is used as, e.g., a circuit element for switching a frequency in an oscillation circuit. One of high-performance variable capacitors is a capacitor using a junction capacitance of a PN junction diode.
For example, Japanese Laid-Open Publication No. 10-74961 discloses a semiconductor device having a PN junction formed by sequential implantation of N-type and P-type impurities. In this semiconductor device, the PN junction serves as a capacitance portion of a variable capacitor.
FIG. 9 is a schematic block diagram of the structure of a conventional device including an oscillation circuit mounted on a semiconductor substrate, an external variable capacitor and the like. As shown in FIG. 9, in the conventional device, the oscillation circuit (which includes a bipolar transistor BT) is provided on a semiconductor substrate 100 within a package. A variable capacitor VAR such as a variable capacitance diode and a resonance inductor Ind are provided on a motherboard (not shown) on which the semiconductor substrate 100 is mounted.
When an oscillator capable of varying a frequency is used, active elements are arranged within the oscillation circuit and the variable capacitor VAR (such as a variable capacitance diode) required to implement a resonant state is arranged outside the package in order to reduce the phase noise and obtain the frequency-varying capability. The variable capacitor VAR and the oscillation circuit in the package are connected to each other through a PAD, package leads and the like. The oscillation circuit, the resonance inductor Ind and the variable capacitor VAR form a resonance circuit.
In a device that includes a resonance circuit including both an oscillation circuit and a variable capacitor for varying the frequency of the oscillation circuit, high-performance passive elements such as a variable capacitor are commonly arranged outside the package having a semiconductor integrated circuit mounted therein. This is in order to obtain the capability to vary the frequency in a wide range and to reduce the noise.
In general, a variable capacitor having a greater change rate of the capacitance has greater capability. The capacitance is commonly changed by the change of the range of the depletion layer formed in the PN junction of the semiconductor layer. In other words, the variable capacitor using the PN junction has a larger capacitance when the depletion layer is smaller, and has a smaller capacitance when the depletion layer is larger. Accordingly, a sufficient depth or width is required for the depletion layer.
However, the conventional device of FIG. 9 has the following disadvantages because the variable capacitor such as a variable capacitance diode is provided outside the package.
The resonance circuit includes a large amount of parasitic capacitance Cpara of a PAD, leads and wires. The overall capacitance of the resonance circuit is equal to the sum of the capacitance of the variable capacitor and the parasitic capacitance Cpara. Since the parasitic capacitance Cpara has a fixed value, the variation range of the oscillation frequency of the resonance circuit is reduced. In order to use a high oscillation frequency, e.g., a high frequency signal of 1 GHz or more, at least one of the inductance of the resonance inductor Ind and the capacitance of the variable capacitor VAR must be reduced. However, even if the capacitance of the variable capacitor is reduced, it is difficult to reduce the overall capacitance of the resonance circuit due to the large parasitic capacitance Cpara. Accordingly, it is becoming increasingly hard to deal with the recent rapid increase in frequency (several gigahertz or higher) of the electronic devices such as portable equipments.
It is therefore preferable to form at least an oscillation circuit and a resonant capacitance portion on a common semiconductor substrate and incorporate the semiconductor substrate into a package. Since a bipolar transistor is mostly used in the oscillation circuit (oscillation element), it is preferable to form a bipolar transistor and a variable capacitor on a common semiconductor substrate. In this case, a collector layer of the bipolar transistor and a variable capacitance diode including a PN junction would be formed substantially in a common semiconductor layer.
However, in response to the recent increase in frequency, the collector layer of the bipolar transistor used in the oscillation circuit is increasingly becoming thinner. Accordingly, even in a capacitor that includes a P-type layer and an N-type layer in a semiconductor layer formed simultaneously with the collector layer, it is difficult from the standpoint of the manufacturing process to ensure a sufficient depth or width for the depletion layer formed in the PN junction. In other words, it is difficult to reduce the overall capacitance of the resonance circuit.
It is an object of the present invention to provide a semiconductor device including a variable capacitor having a large variation range of a capacitance and a manufacturing method thereof.
According to one aspect of the present invention, a semiconductor device includes a variable capacitor. The variable capacitor includes a first semiconductor layer of a first conductivity type, and a second semiconductor layer epitaxially grown on the first semiconductor layer. A PN junction region serving as a variable capacitance is formed at a boundary between the first semiconductor layer and the second semiconductor layer.
In this structure, a depletion layer formed at the PN junction extends in a variable range according to application of a voltage. Therefore, a variable capacitor having the depletion layer as a capacitance portion is obtained. The depletion layer can extend entirely across the depth of the first semiconductor layer. Therefore, the range of the depletion layer is increased as compared to the case where a P-type layer and an N-type layer are formed in the first semiconductor layer. In other words, the variation range of the capacitance can be increased even with a shallow collector region, making it possible to deal with a higher frequency. Moreover, improvement in capability of the variable capacitor (i.e., increase in concentration of the PN junction) enables reduction in the number of variable capacitors required for a certain device. In other words, the number of variable capacitors to be formed on a single semiconductor substrate can be reduced. This enables improvement in integration of the semiconductor device.
Preferably, the above semiconductor device further includes a bipolar transistor. The bipolar transistor has a third semiconductor layer, and a fourth semiconductor layer epitaxially grown on the third semiconductor layer. The fourth semiconductor layer has a base layer epitaxially grown simultaneously with the second semiconductor layer. This enables the variable capacitor and the bipolar transistor having excellent high-frequency characteristics to be formed on the same semiconductor substrate.
Preferably, the first semiconductor layer is a Si layer, and the second semiconductor layer is a Si1-x-yGexCy layer (0 less than x less than 1, 0 less than y less than 1). As a result, excellent characteristics can be obtained using a hetero junction.
Preferably, the above semiconductor device further includes an oscillation circuit, and the variable capacitor is connected to the oscillation circuit. As a result, a resonance circuit capable of adjusting a frequency in an excellent manner can be obtained.
According to another aspect of the present invention, a method for manufacturing a semiconductor device including a variable capacitor and a bipolar transistor on a common semiconductor substrate includes the steps of: (a) forming a first semiconductor layer of a first conductivity type in a region of the semiconductor substrate where the variable capacitor is to be formed, and forming a second semiconductor layer of the first conductivity type in a region of the semiconductor substrate where the bipolar transistor is to be formed; and (b) after the step (a), forming a third semiconductor layer on the first semiconductor layer of the semiconductor substrate and forming a fourth semiconductor layer on the second semiconductor layer both by an epitaxial growth method.
The above method enables a semiconductor device including a variable capacitor and a bipolar transistor on the same semiconductor substrate to be formed by a reduced number of steps.
Preferably, a collector diffusion layer of the bipolar transistor is formed in the step (a), and a base layer of the bipolar transistor is formed in the step (b).
Preferably, the third and fourth semiconductor layers each including a Si1-x-yGexCy layer (0 less than x less than 1, 0 less than y less than 1) are formed in the step (b). This enables formation of a semiconductor device including a bipolar transistor having excellent frequency characteristics and a variable capacitor having a large variation range of capacitance.